Arm rest and seat switches are commonly used as operational state sensors for work machines such as lawn tractors, skid steer loaders, integrated tool carriers, material handling machines, backhoe loaders and the like. As such, these switches are typically used to sense and detect when the machine operator is properly located in the machine prior to allowing movement thereof and/or enabling and/or disabling various systems associated with such machines. In the case of certain types of work machines such as a skid steer loader, from a safety standpoint, it is advantageous and necessary to ensure that the machine operator is, in fact, seated in the seat with the arm rest lowered to its operative or restraining position prior to starting the engine, and prior to engaging or activating the drive transmission, the implement lift and tilt mechanisms, and/or certain auxiliary hydraulic control systems and other functions. In these types of machines, such switches are commonly incorporated into an operator interlock control system whereby movement of the work machine and/or activation of some or all of the above-referenced systems are prevented unless certain operator safety parameters are met, namely, the operator is properly seated in the operator seat and the arm rest is down and in its restraining position. When so used, the operational state of the machine typically is detected by an arm rest/seat switch circuit configuration wherein the switches are coupled in a parallel arrangement to an electronic controller for sending input signals to such controller regarding the state or condition of such switches. A typical switch/sensor arrangement of this type is disclosed in U.S. Pat. No. 5,711,391 and is illustrated in FIG. 1.
Based upon input signals received from the arm rest and seat switches, the electronic controller will analyze such inputs and thereafter provide appropriate output signals to accomplish various tasks. Since arm rest and seat switches have been wired in parallel to electronic controllers used in work machines, such circuit configuration results in a single line input for each respective switch to the electronic controller. This is clearly illustrated in FIG. 1 wherein two independent switches 12 and 14 are coupled directly to an electronic controller 20, switch 12 representing the arm rest switch and switch 14 representing the seat switch. In this parallel switch arrangement, arm rest switch 12 is connected to controller 20 via conductive path 16 and seat switch 14 is connected to controller 20 via conductive path 18. In this particular circuit arrangement, latent faults in the circuit that result in normal operation will go undetected. For example, referring to FIG. 1, shorts to ground or open circuits on either the arm rest switch 12 or the seat switch 14 will go undetected as these failures look like a normal operating condition to controller 20. More particularly, a short to ground on arm rest switch 12 will look to controller 20 as if switch 12 is closed thereby indicating that the arm rest is down and the machine is safe to operate regardless of the actual state or position of the arm rest. Similarly, an open circuit on switch 12 will look to controller 20 as if the arm rest is always up and the seat switch 14 failing in its closed position will appear to controller 20 as though the machine operator is properly positioned in the seat.
As a result, the prior art circuit configuration 10 does not always identify certain failures and when certain faults are diagnostically recognized, it is often times difficult to determine if a switch has failed or if a failure has occurred in the wiring harness such as conductive paths 16 and 18. Additional troubleshooting of the system and circuit must therefore be accomplished in order to further isolate and detect the particular failure which has occurred, which additional action increases costs and machine downtime.
Improved diagnostic capability is therefore desirable. It is also desirable to build some redundancy into the arm rest/seat switch circuit configuration such that the input signals received from these switches are somehow tied together so as to give a better indication to the electronic controller that the operator is in fact properly located in the machine seat with the arm rest in its down and operative position.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.